1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to control of rotation driving of an image carrier or the like.
2. Related Background Art
A high-speed color image forming apparatus is conventionally known, in which images formed by a plurality of image forming units are overlappingly transferred to a conveyed recording sheet.
An apparatus of this type suffers the following problem. That is, due to mechanical precision, the moving irregularities of a plurality of photosensitive drum and conveyor belt occur and the relationship in moving amount between the conveyor belt and the outer surfaces of the photosensitive drums varies at the transfer positions of the respective image forming units in units of colors. For this reason, the overlapped images of the respective colors do not match resulting in color misregistration (position error).
The color misregistration is mainly classified into shifts in the main scanning and subscanning directions. The shifts include a steady error caused by a constant shift within one image and an unsteady error caused by a periodically varying shift.
The steady error can be conventionally corrected as follows. Position error correction marks are formed on the conveyor belt in units of colors, and position errors are detected on the basis of mark read results, thereby correcting the steady error.
The unsteady color misregistration and particularly misregistration caused by irregular convey operation may be corrected as follows. The rotation states of the photosensitive drums and conveyor belt are monitored, and rotation driving of the motor is so controlled as to cancel the rotation errors and make the rotational speed constant.
Conventionally, rotation driving control data obtained at the time of manufacture are stored in a memory. Rotation driving of the motor is controlled in actual image formation on the basis of the stored control data.
Conventionally, since the control data for canceling rotation errors are stored in the memory in advance, as described above, the control data cannot cope with changes in rotation errors caused by changes in environment of the apparatus and changes with age.
Unsteady shifts occur due to variations in rotation errors, and an excellent image cannot be obtained.
It is an object of the present invention to solve the conventional problems described above.
It is another object of the present invention to eliminate rotation errors of rotary members in an image forming apparatus to obtain a high-precision image.
In order to achieve the above objects according to an aspect of the present invention, there is provided an image forming apparatus comprising:
image forming means including an image carrier and write means for forming an image on the image carrier;
a moving member which moves to transfer, at a transfer position, the image formed on the image carrier;
driving means for rotating and driving the image carrier; and
control means for controlling rotation driving of the driving means on the basis of a rotation state of the image carrier in a predetermined interval centered on the same rotation position of a preceding rotation as that of a current rotation.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the accompanying drawings.